FIG. 1 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
Gas turbine engines must be able to withstand several failure modes before being certified for use on commercial airlines. One of these failure modes is an extreme event known as a fan blade off in which a blade of the fan is released in operational service.
As will be appreciated, when a fan blade detaches there is a high energy impact on the fan casing as the blade moves outwards. Also, the fan assembly becomes significantly unbalanced due to the loss of the fan blade and the new asymmetrical loading around the fan's axis of rotation. The imbalance can create large orbiting radial forces which need to be accommodated to prevent the engine breaking up, particularly at high rotational speeds.
The present invention seeks to provide a support structure which collapses in a specific and predetermined way such that the radial loads of the fan can be redistributed and the fan allowed to orbit about a new centre of mass.